1. Field of the Invention
The present invention relates to a hydraulic control apparatus such as a construction machine, for example, a hydraulic shovel or the like, provided with a plurality of hydraulic actuators, and more specifically, relates to the hydraulic control apparatus which can selectively conduct a bleed off function in a way that in the case of operating a relatively heavy load, a plurality of actuators are simultaneously operated, and in the case of operating a relatively light load, only one actuator is operated at the same time, wherein in the case of the light load, a failure due to occurrence of hunting is prevented by conducting a bleed off function, and in the case of the heavy load, performance of a hydraulic control valve is elevated by suppressing the bleed off function, thereby improving the overall operation efficiency.
2. Description of Related Art
FIG. 5 illustrates a conventional hydraulic control circuit diagram on which the present invention is based directly. (Japanese laid open patent publication 2002-295405)
In order to make it easy to understand the hydraulic control circuit diagram illustrated in FIGS. 2 and 3 with regards to the embodiments of the present invention, a detailed explanation of FIGS. 5, 6, and 7 follows.
In FIG. 5, a reference numeral 1Z is a variable-capacity pump. A delivery line 17 of the pump branches to supply lines 18Z and 19Z, and a first change over valve group (hereinafter: change-over valve group A) is connected to the variable-capacity pump 1Z through the supply line 18Z. A second change-over valve group (hereinafter: change-over valve group B) is connected to the variable-capacity pump 1Z through the supply line 19Z and a pressure compensation flow control means (hereinafter: block C). Change-over valves A-1 and A-2 in the change-over valve group A are connected through the supply line 18Z to the delivery line 17Z in parallel. Split flow compensation valves 26Z and 27Z are arranged respectively between the return lines 28Z and 29Z of each change-over valve A-1 and A-2, and tank lines 30Z and 31Z. The hydraulic oil from the tank lines 30Z and 31Z are discharged to a tank 38Z. Functions or roles of the change-over valves and the split flow compensation valves in the change-over valve group A are described below.
In FIG. 5, for example, when only the change-over valve A-2 is operated in the direction of left in the drawing, the pressurized hydraulic oil of a supply line 187 is supplied to an arm hydraulic cylinder 56Z provided in a hydraulic shovel through a check valve 527Z and passages 53Z, 54Z, and 55Z, through a throttle 50Z provided in the change-over valve A-2.
At the same time, the hydraulic oil from the arm hydraulic cylinder 56Z through passages 57Z, 58Z, and a return line 29Z reaches the split flow compensation valve 27Z, and then returns to the tank 38Z through the tank line 31Z.
In the present conditions, the pressurized hydraulic oil after passing the throttle 50Z can branch at the upstream of the check valve 52Z, and then, this branched passage can be further branched to a passage 397 and a passage 40Z. The passage 39Z is connected to a load sensing line 25Z through a check valve 45Z. The hydraulic oil pressure on the passage 40Z, together with a spring 44Z, acts in the direction of opening on the split flow compensation valve 27Z and the hydraulic oil pressure on the load sensing line 25Z acts in the direction of closing on the split flow compensation valve 27Z through a passage 59Z.
Since only the change-over valve A-2 is operated, the hydraulic oil pressure on the passage 40Z which acts in the direction of opening on the split flow compensation valve 27Z is equal to that on the passage 59Z which acts in the direction of closing. Also because the spring force of the spring 44Z is acting in the direction of opening on the split flow compensation valve 27Z, the split flow compensation valve 27Z is held in the state of full opening.
On the other hand, a bypass line 37Z is branched from the supply line 18Z, and a pressure regulating means 3Z and a pressure generating means 4Z are arranged on the bypass line 37Z.
On the pressure regulating means 3Z there is the force acting in the direction of closing, caused by the spring force of a spring 48Z, and by the hydraulic oil pressure on the load sensing line 25Z through the load sensing line 47Z, and also the hydraulic oil pressure on the supply line 18Z is acting in the direction of opening on the pressure regulating means 3Z through a passage 49Z.
Furthermore, a passage 5Z can branch from a portion at the downstream of the pressure regulating means 3Z on the bypass line 37Z and at the upstream of the pressure generating means 4Z. The hydraulic oil pressure on this passage 5Z acts on a delivery flow adjusting device 2Z for the variable-capacity pump 1Z, thereby controlling the amount of delivery flow of the variable-capacity pump 1Z in the fashion of the negative feedback control method. Accordingly, in such a configuration the degree of opening of the pressure regulating means 3Z is described below.
The hydraulic oil pressure at the upstream of the throttle 50Z in the change-over valve A-2 acts in the direction of opening on the pressure regulating means 3Z through the passage 49Z.
On the other hand, in the direction of closing of the pressure regulating means 3Z, the spring force of the spring 48Z and the hydraulic oil pressure at the downstream of the throttle 50Z in the change-over valve A-2 through the load sensing line 47Z act. That is, the pressure regulating means 3Z is adjusted to the degree of opening with which the force caused by the differential pressure before and behind the throttle 50Z in the change-over valve A-2 and the spring force of the spring 48Z balance, and then the hydraulic oil at the flow rate corresponding to the degree of opening flows to the pressure generating means 4Z.
Accordingly, at the upstream of the pressure generating means 4Z, the hydraulic oil pressure corresponding to the above-mentioned flow rate occurs. This hydraulic oil pressure acts on the delivery flow adjusting device 2Z through the passage 5Z, and the delivery quantity of the variable-capacity pump 1Z is adjusted. Since the differential pressure before and behind the throttle 50Z in the change-over valve A-2 becomes fixed regardless of the load pressure of the arm hydraulic cylinder 56Z, when the degree of opening of the throttle 50Z is held constant, the degree of opening of the pressure regulating means 37 becomes fixed.
Thus, the degree of the opening of the pressure regulating means 3Z will be dependent only on the degree of the opening of the throttle 507 of the change-over valve A-2.
The degree of opening of the throttle 50Z of the change-over valve A-2 changes with the control inputs to it. Therefore, pressure which acts on the delivery flow adjusting device 2Z causes the hydraulic oil quantity supplied to the arm hydraulic cylinder 56Z from the variable-capacity pump 1Z to be adjusted by the control input to the change-over valve A-2 regardless of the load pressure of the arm hydraulic cylinder 56Z.
Next, assuming that the change-over valve A-1 is operated in addition to the change-over valve A-2, and the load of a boom cylinder 70Z which is connected to the change-over valve A-1 is larger than that of the arm hydraulic cylinder 56Z, the load pressure of the boom cylinder 70Z connected to the change-over valve A-1 acts on the load sensing line 25Z through a path 41Z and a check valve 46Z.
Since the load of the boom cylinder 70Z connecting with the changeover valve is larger than that of the arm hydraulic cylinder 56Z, the pressure acts in the direction of closing on the split flow compensation valve 27Z in the side of light load (i.e., the arm hydraulic cylinder 56Z) through a passage 59Z, and the observed pressure of the arm hydraulic cylinder 56Z rises. As a result, in both change-over valves, the differential pressure at the throttles 50 and 51 becomes equal, and then those hydraulic actuators connected to the change-over valves A-2 and A-1 can be operated simultaneously even when those loads differ.
Next, block C defined herein as a pressure compensation flow control means and the change-over valve group B defined herein as the second change-over valve group, both of which are connected at the downstream of the block C will be explained.
The supply line 19Z which is branched from the delivery line 17Z of the variable capacity pump 1Z is arranged in the block C, and an open and close motion valve 8Z is arranged at the downstream of the line 19Z.
A compensation valve 22Z is arranged at the downstream of the open and close motion valve 8Z. Further, the hydraulic oil pressure at the downstream of the above-mentioned valve 8Z and the upstream of the compensation valve 22Z is supplied through a passage 23Z and a check valve 24Z to the load sensing line 25Z of the change-over valve group A. The hydraulic oil pressure at the upstream of the open and close motion valve 8Z acts on the valve 22Z in the direction of closing through a passage 20Z.
On the other hand, the hydraulic oil pressure at the downstream of the valve 8Z is acting on the valve 22Z in the direction of closing together with the spring force of a spring 9Z through the passage 23Z and a passage 66Z.
In addition, the open and close motion valve 8Z intercepts the passage from the supply line 19Z to the compensation valve 22Z, when it is in the neutral position by the spring force of a spring 21Z.
The change-over valve group B is arranged in the downstream of the block C, and the hydraulic oil passing through the compensation valve 22Z is supplied to a supply line 60Z of the change-over valve group B.
In FIG. 5, change-over valves B-1 and B-2 which constitute the change-over valve group B, are of the ordinary type of the open-center, and the hydraulic oil supplied to the supply line 60Z is discharged to the tank 38Z through center-bypass passages 61Z and 32Z, when each of the change-over valves B-1 and B-2 are in the neutral position.
Furthermore, the change-over valve B-1 is connected to the pilot valve which is not illustrated, through signal lines 13Z and 14Z, and signal lines 13Z and 14Z are connected to the opening side of the open and close motion valve 8Z through a shuttle valve 15Z and through the signal line 11Z.
Next, with regards to the block C and the change-over valve group B, operation will be explained based on FIGS. 5, 6 and 7, based on when only the changeover valves in the change-over valve group B are actuated.
When the pilot valve (not illustrated) is operated, and then a hydraulic oil pressure signal is applied through a hydraulic oil pressure signal line 13Z to the change-over valve B-1, the change-over valve B-1 moves to the left in FIG. 5.
At the same time, the hydraulic oil pressure signal of the signal line 13Z acts on the open and close motion valve 8Z through a shuttle valve 15Z and the signal line 11Z, and the passage in the valve 8Z changes from a full closing position to a throttling position. That is, the open and close motion valve 8Z is interlocked with the control input of the change-over valve B-1, and the degree of opening is adjusted.
In addition, as shown in FIG. 7, the degree of opening of a throttle 64Z in the open and close motion valve 8Z is usually set up so that the opening of the throttle 64Z may increase gradually in accordance with the rise of the hydraulic oil pressure on the signal line 11Z. When the hydraulic oil pressure signal is applied to the signal line 13Z so that the change-over valve B-1 and the open and close motion valve 8Z are operated, and then the change-over valve B-1 moves, the supply line 60Z is connected to a revolution motor 34Z through a check valve 62Z, and the return hydraulic oil from this revolution motor 347 will be discharged through a return line 65Z to the tank 38Z via the passage which corresponds in the location of (X) indicated in FIG. 6. The hydraulic oil quantity supplied to the supply line 60Z through the block C from the supply line 19Z is determined by the degree of opening of the throttle 64Z in the valve 8Z, and the degree of opening of the compensation valve 22z. The degree of opening of the compensation valve 22Z is described below.
The hydraulic oil pressure at the upstream of the throttle 64Z in the valve 8Z acts on the compensation valve 22Z in the direction of closing. On the other hand, the hydraulic oil pressure at the downstream of the throttle 64Z in the valve 8Z, and the spring force of the spring 9Z act on the compensation valve 22Z in the direction of opening. That is the compensation valve 227 tends to be closed by the differential pressure (i.e., differential pressure before and behind the throttle 64) at the upstream of the throttle 64 of the valve 8Z, and at the downstream thereof, and tends to be opened by the spring force of the spring 9Z.
Accordingly, regulating the operation of the degree of opening of the compensation valve 22Z is automatically carried out based on the degree of opening corresponding to the differential pressure applied before and behind the throttle 64Z and the spring force of the spring 9Z balance.
Therefore, regardless of the working hydraulic pressure to the actuator, the degree of opening of the compensation valve 22Z is dependent only on the degree of opening of the open and close motion valve 8Z i.e., the control input to the change-over valve B-1.
On the other hand, the hydraulic oil pressure at the downstream of the open and close motion valve 8Z is led to the opening side of the pressure regulating means 3Z through a passage 23Z, a check valve 24Z, and the load sensing lines 25Z and 47Z, and the pressure at the upstream of the throttle 64Z in the open and close motion valve 8Z is led to the opening side of the pressure regulating means 3Z through supply lines 18Z, 19Z and a passage 49Z.
Accordingly, the amount of delivery flow from the variable-capacity pump 1Z is adjusted so that the pressure drop (i.e., differential pressure before and behind the throttle 64Z) which depends on the throttle 64Z in the open and close motion valve 8Z may balance with the spring force of the spring 48Z. When the hydraulic oil pressure of the signal line 13Z rises, the hydraulic oil quantity supplied to the change-over valve group B from the block C increases, because the degree of opening of the throttle 64Z in the open and close motion valve 8Z will increase.
Further, because the change-over valve B-1 is of the type of the open-center, the operability can be smooth since the degree of opening of the passage to the revolution motor 34 gradually increases, while the center-bypass passage is gradually closed in accordance with the control input as shown in FIG. 6. In other words, the amount of supply of the hydraulic pressure oil supplied to the revolution motor 34Z increases in accordance with the incremental increase in the control input to the change-over valve B-1.
Accordingly, the revolution motor 34Z can drive a hydraulic actuator having very large inertia, such as a revolution motor provided in the hydraulic shovel, without an occurrence of a jumping out feeling at the starting time, because hydraulic oil pressure compensation is always carried out by the aid of a load sensing function. Thus, a smooth starting performance can be achieved.
As mentioned above, while applying the load sensing function to the actuators connected to the change-over valve group A in accordance with the present invention, the function of the open-center can be applied, without using another pump for the actuators connected to the changeover valve group B. That is, the actuator suitable for connection with a change-over valve provided with the load sensing function and the actuator suitable for connection with a change-over valve provided with the function of the open-center can be both driven by a common variable-capacity pump.
Accordingly, even when two or more actuators provided in the hydraulic shovel, are operated simultaneously, each of the actuators can differ in inertia and load pressure. The optimal operation for the property of each actuator can be performed, and in addition, the problem of the increased cost caused by arranging another pump for revolution operations and the problem that the layout of control equipment pose difficulties that need solutions.
However, further improvement with regards to the hydraulic control circuit illustrated in FIG. 5 may be achieved, because its performance is largely affected by the load when driving an actuator with a relatively heavy load of inertia, such as the revolution in the construction machinery as a hydraulic shovel, which makes use of the above mentioned hydraulic control circuit. A hunting phenomenon can occur in the hydraulic circuit and the performance of the actuator can become clumsy, particularly wider the condition of a small revolution orientation, i.e., the boom is oriented nearly to the vertical direction. As a result, the moment of inertia can be small, and the control input is fed back through the operator who manipulates the handling lever of the hydraulic shovel.
To eliminate the hunting phenomenon, a bleed off approach is generally proposed that prevents the hydraulic oil pressure from rising sharply by having a control valve arranged between an actuator and a tank. For example, a directional change over valve is opened while the actuator is operated, thereby partially discharging the pressurized hydraulic oil in the hydraulic circuit to the tank.
The bleed off, which refers to discharging pressurized hydraulic oil to a tank, is effective in eliminating a hunting phenomenon that is caused by a heavy load and a sharp rise of the hydraulic oil pressure. However, discharging the pressurized hydraulic oil to the tank lowers the hydraulic pressure and flow rate in the hydraulic circuit, and as the result, induces lowering of operation efficiency.
On the other hand, when there is relatively low demand for the flow rate in the hydraulic circuit, such as the sole operation for revolution, no remarkable difference in the hydraulic pressure or the flow rate is noticed between bleeding off and the absence of bleeding off.
But, when demand for the flow rate of pressurized oil increases due to a complex operation of combining the heavy load caused by lifting heavy goods with the revolution operation, the difference in the flow rate or in the hydraulic pressure becomes remarkable. In such a condition it follows that the operation efficiency itself becomes worse when bleeding off as mentioned above.
Furthermore, a hydraulic oil passage may be formed between a spool of a valve body and inside of the valve body so as to enable to bleed off within the change over valve, which controls the pressurized oil supply and discharge to and from a designated hydraulic actuator. As the result, this passage formed for bleeding off conducts the bleed off, even when demand for the flow rate of pressurized oil increases because of the complex operation of combining the heavy load with the revolution operation, causing the operation efficiency to become worse.
Thus, an object of the present invention is to provide a hydraulic control apparatus capable of preventing failure due to occurrence of hunting; to improve the performance of a hydraulic control valve; and to improve operation efficiency by effectively conducting the bleed off.
Here, the above “effectively conducting the bleed off” means that the bleed off is selectively conducted in accordance with a state of operation in the hydraulic circuit, such that when there is a relatively small demand for the flow rate in the hydraulic circuit, such as the sole operation for revolution, the bleed off is conducted so as to suppress the hunting. When the demand for the flow rate increases in the hydraulic circuit because of the complex operation of combining the heavy load with the revolution operation, the bleed off is ceased.